1. Field of the Invention
The present invention is generally concerned with contact lenses, that is to say lenses designed to be placed directly on the cornea of the eye.
2. Description of the Invention
Overall, a contact lens comprises a rear surface, a front surface, and a peripheral edge surface extending between the front and rear surfaces, defining with these the peripheral edge portion of the lens.
As is well known, two of the functional constraints to be met by a contact lens are at least partially dependent on the geometrical configuration of this edge portion: good adaptation to the eye of the user and an acceptable level of comfort for the user.
Previously, it has been generally accepted that for contact lenses, and in particular for flexible contact lenses such as those manufactured of silicone, for example, meeting these two functional constraints implied the use of a peripheral edge portion progressively tapering off to quasi zero thickness.
This is the case in French Patent No. 2 399 043 (filed as application No. 78 21 655 on July 21 1978), for example.
When a contact lens has a peripheral edge portion which progressively tapers, its peripheral edge surface extends generally obliquely and rearwardly, from the rear surface to the front surface, to follow as closely as possible the sclera of the eye while forming with the latter a tapering space intended to facilitate the introduction under the lens of a new film of lacrimal fluid each time the eyelid blinks, to secure the aforementioned good adaptation.
Good adaptation and acceptable comfort are usually obtained with this configuration of the peripheral edge portion.
With some contact lenses, however, and particularly those manufactured of silicone, this configuration of the peripheral edge portion frequently leads to a "sucker effect" whereby the lens adheres tightly to the cornea.
This is a major disadvantage in that, on the one hand, it opposes satisfactory circulation of lacrimal fluid between the lens and the cornea, necessary in order to renew the film of lacrimal fluid providing the required good adaptation, and in that, on the other hand, it may lead to problems with removing the lens at the end of the day.
In French Pat. No. 2 416 104 (filed as application No. 78 03323 on Feb. 7, 1978) and the corresponding European application No. 003695, one of a number of molding configurations disclosed is such as to produce on the contact lens concerned a plane peripheral edge surface extending substantially perpendicular to the axis of the lens.
Apart from this theoretical arrangement of a molding device, there is no suggestion anywhere in this French Patent application that the corresponding configuration of the lens is apt to overcome the problem of the "sucker effect".
In French Patent No. 2 327 562 (filed as application No. 76 29795 on Oct. 4, 1976) a contact lens is disclosed in which the peripheral edge surface is cylindrical and may therefore be regarded as lying wholly within that portion of space situated to the front of a reference surface which extends in a generally transverse direction relative to the axis of the lens and is tangential to the lens in the area in which the rear surface joins the peripheral edge surface.
In practice, the resulting shape of the edge portion of this type of contact lens is not apt to overcome the problem of the "sucker effect", which is nowhere mentioned in this French Patent application.
The general objective of the present invention is to provide a contact lens with which the "sucker effect" may be overcome and providing good adaptation and an acceptable level of comfort for the user.